A water absorbent resin (SAP/Super Absorbent Polymer) is a water swellable and water insoluble polymeric gellant, which is widely and mainly used in disposable articles including an absorbing article such as a disposable diaper, a sanitary napkin, an agricultural and horticultural water retaining agent, and an industrial water stopping agent. As a material for the water absorbent resin, varieties of monomers and hydrophilic polymers have been proposed. Among them, a polyacrylic acid (salt)-type water absorbent resin, which is formed by using acrylic acid and/or a salt thereof as a monomer, is widely used in industrial purposes due to its high water absorption performance. For polymerization of acrylic acid, a radical polymerization initiator such as persulfate salt, hydrogen peroxide, organic peroxide, water extractable azo initiator, and UV polymerization initiator is generally used. Among them, the radical polymerization using sodium persulfate or the like is disclosed in the following Patent Literature or the examples included therein.
The water absorbent resin can be obtained by micronizing a hydrogel crosslinked polymer, which is obtained by polymerizing an aqueous monomer solution, either during or after polymerization, then, drying the particulate hydrogel crosslinked polymer obtained (Non-Patent Literature 1).
As for the method of drying water absorbent resin, a method of using a belt type dryer (Patent Literatures 1 to 5), a method of thin film drying by using a drum dryer or the like (Patent Literature 6), a method for azeotropic dehydration in an organic solvent (Patent Literature 7), a method for drying in fluidized bed (Patent Literature 8), a method for drying in a bed fluid ized by vibrating (Patent Literature 9), and a method for drying under stirring by using a rotor (Patent Literature 10), or the like are known.
As a condition for drying water absorbent resin, a method of controlling dew point or temperature (Patent Literatures 11 and 12) and a method of crushing during drying to dry under stirring (Patent Literature 13), or the like have been suggested for improving physical properties (for example, reducing residual monomers, increasing water absorption capacity, and reducing water extractable components).
Further, since non-dried products may occur during drying of a water absorbent resin to cause excessive load on crushing, a method for removing non-dried products is also known (Patent Literatures 14 to 16). To prevent occurrence of non-dried products, a method of regulating fluidity of a polymer gel (Patent Literature 17), a method of using a gel floating device in a dryer (Patent Literatures 18 and 19), a method of drying by using a specific apparatus for supplying constant quantity of a gel to a dryer (Patent Literature 20), a method of using infrared or the like in combination with hot air (Patent Literature 21) are known. Further, for improvement of drying efficiency, a method of adding a surfactant or an inorganic fine particle to hydrogel (Patent Literatures 22 to 26) is also known. A drying method for hydrogel with low neutralization rate is also known (Patent Literature 27). Still further, a method of drying at 100 to 250° C. with the index of thermally decomposable radical polymerization initiator content at 40 to 100 in a hydrogel before drying is known (Patent Literature 28).